c++: Fix indirect partitions [PR 98944]

c++: Fix indirect partitions [PR 98944]

The most recent reimplementation of module loading initialization
changed the behaviour of setting an import's location, and broke some
partition handling.

	PR c++/98944
	gcc/cp/
	* module.cc (module_state::is_rooted): Rename to ...
	(module_state::has_location): ... here.  Adjust callers.
	(module_state::read_partitions): Adjust validity check.
	Don't overwrite a known location.
	gcc/testsuite/
	* g++.dg/modules/pr98944_a.C: New.
	* g++.dg/modules/pr98944_b.C: New.
	* g++.dg/modules/pr98944_c.C: New.
	* g++.dg/modules/pr98944_d.C: New.